The invention relates to skid steer machines, and more especially to compact skid steer loaders.
Skid steer machines are commonplace in the field of construction equipment machinery. A skid steer machine can be defined by the fact that it comprises at least a left propulsion unit and a right propulsion unit which can be controlled independently in terms of their propulsion speed and of their propulsion direction, i.e. whether they operate forwardly or rearwardly. Therefore, on such machines, the left or right movements of the machine come as a consequence of a difference in the speed of the left and right propulsion units, and those machines can therefore be dispensed of any dedicated steering mechanism such as deflectable wheels.
One major advantage of skid steer machines is that they can easily achieve on-the-spot turns which make them very maneuverable. In the field of compact construction equipment machines, excavators and loaders are often built as skid steer machines. Skid steer excavators usually have propulsion units equipped with endless tracks. On the other hand, compact skid steer loaders are most often equipped with two wheels on each side of the machine, each pair of wheels on one side of the machine being non-deflectable and driven synchronously by the same propulsion unit, although skid steer loaders equipped with endless tracks are also found.
Whereas excavators are mostly static machine, that is to say that in a typical working cycle the machine is not significantly displaced, skid steer loaders are essentially machines which are used to carry loads from one place to another. Therefore, the typical work cycle of a loader is to pick up a load, to move from a first location to a second location, to discharge the load, and to come back from the second location to the first location to pick up the next load. Therefore, if the two locations are at some distance one from the other, there is an evident incentive to be able to travel as quickly as possible from one location to the other. On the other hand, when the machine arrives either at the first location or the second location, it needs to arrive slowly to insure a precise handling of the machine. Also, when carrying heavy loads on steep grades, the power needed for displacing the machine might become important with respect to the available power output of the engine driving the machine, especially in compact machines where only a small engines can be fitted.
Skid steer machines are most often equipped with so-called hydrostatic transmissions. More precisely such transmissions comprise a variable capacity pump which is hydraulically connected in closed-circuit to a hydraulic motor. In some cases, the hydraulic motor is of the variable capacity type but, the hydraulic motor is often of a fixed capacity type. For a given pressure of fluid delivered by the pump, which will determine the torque delivered by the motor, and for a determined capacity of the motor, the speed of the motor will be directly linked to the flow of hydraulic fluid delivered by the pump to the motor. One can determine an operating ratio as being the ratio between the propulsion speed of the propulsion units and the flow of hydraulic fluid delivered by the pump to the motor.
It is known in the industry to have means for varying this operating ratio. For example, the hydraulic motor could be of the type having a variable capacity. Another possibility is to have some kind of a mechanical gearbox between the motor and the wheels or the endless track of the propulsion unit. Into more simple arrangement, there are commercially available hydraulic motors which have inherently a fixed capacity but wherein the displacement of the motor can nevertheless be easily changed. An example of such motors is a radial piston motor where the pistons can be partially deactivated. An example of such motors is the MCR range of radial piston motors available from the Bosch Rexroth company. In those motors, a fluid pressure signal can be used to deactivate partly the pistons of the motor so as to in effect divide by two the instantaneous effective capacity of the motor. By dividing the motor capacity by a factor of two, it is possible, for the same of fluid pressure delivered by the pump, to double the speed of the motor. On the other hand, this decreases also by a factor of two the mechanical torque available on the output shaft of the motor. It should be mentioned the dividing factor may be other than two.
Therefore, in the context of a skid steer machine it is desirable to provide the propulsion units with the means to change the operating ratio depending on which parts of its work cycle it is operating in. On the other hand, this should be achieved through a system both inexpensive and easy to operate for the machine's operator.
An aspect of the invention provides for a skid steer machine having separate left and right hydraulic propulsion units, wherein the machine comprises an operating ratio change system by which the ratio of the propulsion speed of at least one of the hydraulic propulsion units versus the hydraulic flow directed to the propulsion units can be changed from at least a first ratio to a second ratio and inversely, characterized in that the operating ratio change system comprises an operating ratio change ECU which controls an operating ratio change device to operate in at least a first ratio configuration or a second ratio configuration, and in that the operating ratio change system operates in an automatic operating mode where the operating ratio change ECU selects the operating ratio according to machine parameters received by the operating ratio change ECU.